Ink jet printing is a non-impact method of printing that involves ejecting ink from a nozzle onto paper or other print media. The actual ink ejection method may occur via several processes including pressurized nozzles, electrostatic fields, piezoelectric elements within an ink nozzle, and heaters for vapor phase bubble formation.
The composition of the ink is traditionally comprised of deionized water, a water soluble organic solvent, and a colorant. The colorant may be a soluble dye or insoluble pigment. Several problems, however, are associated with soluble dyes that are not applicable to insoluble pigments. These problems include poor water-fastness, poor light-fastness, poor thermal stability, facile oxidation, dye crystallization, and ink bleeding and feathering on the print medium. To circumvent these problems, use of a pigment as the colorant is preferred. Pigments generally have better light-fast and water-fast properties, are more resistant to oxidation, and have higher thermal stability.
With the benefits obtained with pigment based inks, come new problems to overcome. Specifically, there are new problems in photo printing. When pigment based inks are printed on microporous photo paper, the paper acts like a filter in the sense that the bulk of the ink solids stay in the form of a cake on the paper surface. This leads to handling issues. Having the solids on the surface of the paper leaves the colorant exposed to mechanical forces when the photo is handled. Handling can lead to smearing and scratching of the print with current pigment ink formulations.
A specific type of scratch that occurs with current pigment inks is known as green scratch. This type of scratch occurs in gray and black areas of photos. The gray and black areas are composed by layering cyan, yellow, and magenta. When scratched, the cyan and yellow droplets have less mechanical strength than the magenta. This leads to smearing and blending of the cyan and yellow which results in a green color along the scratched area. Logical fixes would be to increase the mechanical strength of the ink cake or to protect the ink cake from mechanical forces by use of a coating or additive that provides sufficient protection.
By changing ink or photo paper formulation, many methods have been disclosed in the past to improve smear resistance. For example, US 20040102541 disclosed graft polymeric dispersants in pigment-based inks to increase smear resistance; US 20050166794 disclosed using a dispersed cellulose ester as a binder additive in pigment-based inks to enhance the durability of the printed images; US 20050134665 disclosed ink additives that contains benzyl methacrylate to increase smear resistance. US 20050110856, U.S. Pat. No. 6,020,397, and U.S. Pat. No. 6,503,307 disclosed some imaging fixing components (reactive liquids) to improve abrasion resistance. On the other hand, U.S. Pat. Nos. 6,844,035, 6,689,433, and 6,528,148 disclosed the use of a poly(vinyl alcohol polyethylene oxide) copolymer and other binders in polymer-based photo papers to improve smear-fastness. U.S. Pat. No. 6,626,531 disclosed the surface roughness from 0.2 to 2.0 μm to improve smear for pigment-based ink on a porous media.
Wax emulsions have been identified as a possible fix for this issue. Waxes are commonly used in ink and coatings formulations. They are known to improve various properties such as slip or lubrication, rub or abrasion resistance and anti-blocking. These properties relate to the handling issues that are observed with current pigment based inks. However, these wax emulsions can adversely affect print quality and/or jetting characteristics.
Shibata et al U.S. Pat. No. 4,724,002 discloses heat-sensitive transfer media containing an ink composition comprising a binder and a pigment. The binder may be a wax and the pigment may be an oil-based pigment or dry color which is suspended within the binder to impart a color to the ink.
Terry et al U.S. Pat. No. 3,406,137 discloses a pressure transfer ink formulation comprising a pigment dispersed in a binder. The binder is comprised of waxes, wax soluble dyes, an adhesive resin and polysiloxane.
Duff et al. U.S. Pat. No. 4,762,734 discloses an ink donor film comprising in a hydrocarbon solution of wax components and/or polymer components, or a mixture of wax and polymer components, dispersed in a polar phase containing dispersed pigment or dissolved dye. The coloring agent may include a dye and/or pigment wherein the dye is an alcohol or hydrocarbon-soluble dye.
Merritt et al. U.S. Pat. Nos. 4,390,369 and 4,484,948 disclose a natural-wax ink jet ink containing at least one natural wax. The wax may be used as a basic fluid vehicle or as an additive to other fluid vehicles. A coloring agent or dye such as an oil or solvent dye may be added to the composition.
Hayashi et al. U.S. Pat. No. 4,636,258 discloses an ink composition comprising a copolymer, a colorant dispersed therein, and wax or resin. The wax may be added to the colorant when dispersed in the copolymer.
Tabayashi et al. U.S. Pat. No. 4,878,946 discloses a hot melt ink for thermal ink-jet printers comprising an oil-soluble dye and additives dissolved in at least one compound that is solid at ambient temperature.
Trimble et al. U.S. Pat. No. 3,353,974 discloses a homogeneous composition for printing inks and various coatings comprising an oil, wax and/or resinous base, and a high concentration of dispersed pigments.
Wong et al. discloses jetting an ink composition comprising an emulsion of an organic phase in a water phase, the organic phase including at least one of oil and wax. The ink compositions preferably contain a pigment or dye, in either or both phases.
There is a need for ink compositions for ink jet printers which not only provide acceptable jetting properties and printing quality, but which also provide improved photo smear, scratch and scuff resistance properties.
It is an objective of this invention to provide pigment inks containing a wax emulsion that have improved photo smear, scratch and scuff resistance properties.